The present invention relates to electronic heating apparatus of the type commonly referred to as a microwave oven, and particularly to microwave ovens which can be placed atop a table or counter and which have a heating cavity with dimensions generally comparable to a wavelength of the microwave energy used. More specifically, the present invention relates to improved microwave ovens which can be operated on standard 120 volt a.c. household current without using a power transformer.
Microwave cooking is a complex problem and the heating characteristics for microwave ovens and for various types of foods have been mathematically determined for only the simplest cases. It is recognized, however, that the ability of a microwave oven to efficiently and uniformly cook a food load is dependent upon the electromagnetic field pattern which is established in the heating cavity. Normally, a microwave oven is adapted to operate at a predetermined ultrahigh frequency and, typically, a conventional microwave oven of arbitrary dimensions is found to have electromagnetic field modes near the desired wavelength. In practice, one of these modes, usually a transverse electric (TE) mode is excited in the heating cavity, and this mode is characterized by both high impedance and low impedance regions in the heating cavity. Generally, the position in the heating cavity in which food is normally placed lies in the high impedance region of the electromagnetic field pattern.
But most food items have a relatively low impedance in the range of 3 to 200 ohms and, typically, the food presents a series impedance with the standing wave impedance in the heating cavity so that heating occurs in the low impedance region of the wave pattern. Thus, when food is placed in the oven in a high impedance region or partially in a high impedance region and partially in a low impedance region, the result is that the food either does not heat or the heating is not uniform.
Heretofore, this problem was attacked by using rotating mode stirrers in the heating cavity to excite a large number of electromagnetic field modes in the hope that the combined effect of these modes would be to produce a more uniform heating pattern. Similarly, in the copending U.S. application Ser. No. 317,206, filed Dec. 21, 1972, now U.S. Pat. No. 3,798,404, issued Mar. 19, 1974, and the copending U.S. application Ser. No. 320,140, filed Jan. 2, 1973, now U.S. Pat. No. 3,823,295, issued July 9, 1974, both assigned to the assignee of the present invention, rotating mode exciters are utilized to excite and couple specific secondary modes to provide a more uniform time-averaged field distribution in the heating cavity. Alternatively, rotating turntable-type shelves have been provided in the heating cavity for supporting the food and moving the food through different regions of the field pattern so as to achieve a more uniform heating of the food. But all of these prior art attempts at improving the uniformity of heating in the microwave oven involve the use of moving parts which add to the complexity, the expense and the difficulty of maintenance of the electronic oven.